1) Field of the Invention
The present invention relates to a developing unit, which is used in copying machines, facsimile machines, and printers, and to a process cartridge and an image forming apparatus.
2) Description of the Related Art
Conventionally, a two-component developing unit is used to develop a latent image formed on an image carrier. In the two-component developing unit, a two-component developer that includes a toner and a carrier is used as a developer. The two-component developing unit includes a developer stirring and conveying member that is disposed in a developer container to stir the two-component developer, thereby frictionally charging the toner. In general, the developer stirring and conveying member has a structure in which spiral screw blades are provided around a rotation axis. The developer is carried on a surface of a developer carrying member that includes a magnetic pole inside, and is supplied to an electrostatic latent image on the image carrier to develop the latent image. A concentration of the toner in the two-component developer changes during use because the toner is consumed during development. Therefore, it is necessary to replenish the toner corresponding to a change in the concentration to keep the concentration constant, thereby maintaining a quality of images to be formed.
However, when a solid image is successively printed with the two-component developing unit, unevenness is sometimes caused in an image. Such unevenness is called screw pitch unevenness, and the screw pitch unevenness is caused due to insufficiently mixed developer. A developer that is returned to the developer container from the developer carrying member after contributing to development has a low concentration of the toner. The developer returned and a developer that is newly conveyed by the developer stirring and conveying member from the developer container are conveyed together to a developer controlling member without being sufficiently mixed. As a countermeasure against such a problem, fins (paddles) are formed at specific positions on the screw blade to improve a stirring effect of the developer stirring and conveying member. However, this method causes fin pitch unevenness. The fin pitch unevenness occurs because the developer returned is held by the fins, and is conveyed to the developer controlling member without being mixed with the developer newly conveyed from the developer container.
Japanese Patent Application Laid-open No. 2002-287472 discloses a developing unit that includes a developer guiding member. The developer guiding member guides the developer returned form the developer carrying member after development to be within a range in which rotation of the developer stirring and conveying member reaches. With the developer guiding member, the developer returned is more likely to be taken out to the developer stirring and conveying member, instead of staying in the developer container. Thus, the developer having a low concentration of the toner, which is returned to the developer container after development, and a developer never used for development are more effectively mixed in the developer container. As a result, occurrence of the screw pitch unevenness can be suppressed.
When a new developing unit or a new process cartridge including a developing unit is installed or replaced with a new developing unit or a new process cartridge, a sealing member is peeled off from a new developer container before installation or replacement. When an image forming apparatus carrying this new process cartridge is driven, a developer in the developer container passes through the developer controlling member, and is conveyed to the developer carrying member. The sealing member prevents degradation of the developer due to exposure to an atmosphere, a foreign material to be mixed into the developer in the developer container. The sealing member also prevents leak of the developer from the developer container during movement or conveyance of the developing unit. For this sealing member, in the developer container, a surface to which the sealing member is adhered should be sufficiently provided to secure hermeticity.
In recent years, a new type of a toner manufactured by polymerization is used. Such toner is formed to have a particle size within a range of 3 micrometers (μm) to 10 μm in high precision, and a shape of the toner can be controlled. On the other hand, this toner tends to have high fluidity due to an amount of an additive. Therefore, the developer container is required to have even higher hermeticity.